Patients with end stage renal disease (ESRD) constitute a small percentage of Medicare beneficiaries but account for a disproportionate amount of the money spent on Medicare. They are rehospitalized far more frequently than other persons, with 36% of patients rehospitalized within 30 days. As the Centers for Medicare and Medicaid Services (CMS) have increasingly focused on reducing 30-day hospital readmission rates in all Medicare patients including those on dialysis, they have started penalizing hospitals for 30-day readmissions. Even though physicians are increasingly incentivized to avoid 30-day rehospitalizations, it is not known if they are preventable in patients on dialysis (or in Medicare patients as a whole) or if some rehospitalizations are potentially beneficial. The causes of 30-day readmissions in patients on dialysis have not been previously described, and to date, no one has attempted to see if some 30-day rehospitalizations might prevent future adverse events in patients. The First Aim is mostly descriptive and will identify the most common hospitalization and rehospitalization pairs of diagnoses. It will also identify diagnosis pairs that are clinically related to each other and those that are not. The Second Aim will take the most common causes of 30-day rehospitalization and identify groups of patients who are likely to be rehospitalized and those who are likely to die within 3 years, in order to risk-stratify patients b mortality and likelihood for rehospitalization. The Third Aim will take patients who are at high likelihood for death and identify groups of patients who may potentially benefit from 30-day readmission by seeing if they have better outcomes than expected. This research will help clinicians and policy makers better understand when 30-day rehospitalizations may be beneficial to patients on dialysis. It will help direct policy makers to introduce policies that taget reductions in inappropriate rehospitalizations and will help physicians understand when to consider rehospitalization as a potential therapeutic treatment.